The evolution of Liberal thought in the context of International Relations (IR) reflects a long and adaptive intellectual tradition that traces its roots to the philosophical transformations of early modern Europe. Liberalism emerged as a response to the rigidities of feudal and absolutist societies, shaped by the Reformation and Enlightenment movements from the 16th century onwards. Early liberal thinkers challenged entrenched religious dogmas and monarchic authority, advocating for a rational, secular, and scientific worldview. The emphasis on individual liberty, reason, and progress underpinned a vision of society where human beings could develop freely within a framework of rights and responsibilities. This philosophical foundation influenced subsequent thinkers and provided the basis for liberal interpretations of international order, where cooperation, law, and institutions could mitigate the anarchic tendencies of the state system.

Classical liberalism, emerging prominently in the 17th and 18th centuries, focused on the protection of individual rights and the limitation of state power, conceptualizing the state as an instrument to safeguard freedom rather than an omnipotent authority. John Locke emphasized natural rights to life, liberty, and property, insisting that governments must be accountable to citizens and that revolution was justified when rulers violated these rights. Similarly, Adam Smith advanced the principles of economic freedom and market interdependence, arguing that free trade and voluntary exchange would promote social welfare and mutual prosperity. Immanuel Kant, in his seminal essay Perpetual Peace, extended liberal thought into the international sphere, positing that the establishment of republican states, adherence to international law, and promotion of economic interdependence could reduce the likelihood of war. Classical liberalism thus forged a domestic analogy for international relations: just as individuals interact peacefully under law and institutions in society, states could coexist through rules, norms, and cooperative mechanisms.

The early twentieth century witnessed the formal application of liberal ideas to IR, particularly in the aftermath of the First World War. This period, often referred to as Idealism or Utopian Liberalism, sought to construct a peaceful international order by promoting democratic governance, international law, and economic cooperation. Figures such as Norman Angell and Woodrow Wilson championed the creation of international institutions, most notably the League of Nations, designed to prevent war and resolve disputes through collective mechanisms rather than force. Liberal theorists argued that peace was not an automatic condition but a construct achievable through rational, deliberate efforts. They emphasized the potential for mutual understanding and negotiation to harmonize interests across states. However, the rise of fascist regimes in Germany and Italy, coupled with the failure of the League of Nations, exposed the vulnerabilities of early liberal thought, paving the way for the emergence of Realist critiques that highlighted the centrality of power, self-interest, and security in international politics.

Post-Second World War liberalism witnessed a revival, marked by a pragmatic recognition of state and non-state actors in international affairs. The creation of the United Nations exemplified the liberal belief in institutional mechanisms to mediate conflict, promote cooperation, and uphold human rights. Postwar liberalism emphasized the significance of technological advancements, economic integration, and transnational networks in fostering a cooperative international order. Scholars like David Mitrany, Ernst Haas, and Robert Keohane contributed to this transformation, conceptualizing how functional cooperation and international institutions could generate stability in a world still shadowed by the experiences of global conflict. In this period, liberalism acknowledged that while states remained central, individuals, groups, and societal organizations increasingly influenced international outcomes, giving rise to sociological, interdependence, and institutional strands of liberal thought.

Sociological liberalism highlighted the role of transnational actors, emphasizing that international relations are shaped not only by state-to-state interactions but also by networks of individuals, organizations, and societies. John Burton’s Cobweb Model illustrated a complex web of overlapping relationships that transcend borders, while Karl Deutsch’s concept of a Security Community argued that heightened societal interconnections foster expectations of peaceful conflict resolution. Sociological liberals contend that wars often result from institutional failures rather than inherent human aggression, suggesting that the proliferation of non-state actors, including NGOs, multinational corporations (MNCs), and advocacy networks, can promote cooperation and reduce conflict. This approach, however, has faced criticism from realists such as John Mearsheimer, who argue that state power remains decisive and that non-state actors cannot override the primacy of national interests.

Interdependence liberalism, as articulated by thinkers like Richard Rosecrance, posits that growing economic, technological, and political ties among states reduce incentives for conflict. The “Golden Arches Theory” proposed by Thomas Friedman suggests that nations integrated through trade are less likely to engage in war, as conflict threatens mutual economic interests. Interdependence liberalism underscores that modernization and globalization create complex linkages, incentivizing cooperation and stability. Yet, critics argue that economic ties alone cannot prevent war, as demonstrated by conflicts between interdependent states and the inequalities generated by global economic structures. Realists contend that security concerns often override economic interests, while neo-Marxists critique the system as perpetuating a core-periphery hierarchy, privileging developed nations at the expense of the global South.

Institutional liberalism, championed by Woodrow Wilson and later scholars such as Robert Keohane and Joseph Nye, emphasizes the role of international institutions in reducing uncertainty, monitoring compliance, and facilitating cooperation. By creating shared norms, regimes, and decision-making frameworks, institutions like the UN, WTO, NATO, and EU help states pursue absolute gains and collective benefits. Institutional liberalism challenges realist assertions that anarchy inevitably leads to conflict, demonstrating that rules and cooperation can mitigate security dilemmas. The theory has proven effective in areas such as global trade, health (WHO), and disarmament, but faces criticism for its limited enforcement powers, lack of transparency, and vulnerability to powerful states’ agendas, as seen in conflicts like Russia–Ukraine or the Afghanistan crisis.

Republican liberalism, particularly the Democratic Peace Theory advocated by Michael Doyle, posits that liberal democracies are structurally and normatively constrained from waging war against each other. This theory emphasizes democratic norms, shared liberal values, and economic interdependence as mechanisms promoting peace. While historical patterns suggest a low probability of inter-democratic war, critics highlight empirical anomalies such as India–Pakistan conflicts and U.S.-led interventions justified on democratic grounds. Scholars like Douglas Gibler argue that peace often precedes democracy, challenging the deterministic claims of democratic peace theorists. Nonetheless, republican liberalism underscores the normative dimension of liberal thought, integrating values, institutions, and economic incentives into the analysis of international relations.

Functionalism, emerging in the aftermath of the world wars, presents another strand of liberal thought, emphasizing technical and economic cooperation as a pathway to broader political integration. David Mitrany argued that collaboration in practical, non-political domains—such as trade, environment, and health—could reduce conflict and build trust among states. Ernst Haas’s concept of the spillover effect suggested that successful cooperation in technical areas could gradually extend to political spheres. Functionalism influenced the formation of supranational entities such as the European Coal and Steel Community, later the European Union, and international organizations like the WHO, demonstrating the potential of cooperative frameworks. Critiques, however, note that functionalism underestimates sovereignty concerns, neglects power politics, and offers limited solutions for conflicts rooted in historical, cultural, or ideological divisions.

The late 20th century saw the rise of neo-liberalism, which sought to adapt liberal principles to a globalized world economy. Neo-liberal thinkers, including C.B. Macpherson, T. Friedman, John Rawls, Francis Fukuyama, and Kenichi Ohmae, emphasized free trade, minimal state intervention, and globalization as instruments for achieving international cooperation. Neo-liberalism argued that the welfare and protectionist state hindered human enterprise, innovation, and interdependence. By promoting economic liberalism and integration, neo-liberals believed that states could achieve peace and prosperity while enhancing human rights and individual freedoms. Institutions such as the IMF, World Bank, WTO, NAFTA, and APEC became central to implementing neo-liberal ideas, facilitating economic interdependence and global governance. The post-Cold War era further reinforced neo-liberal claims, highlighting the potential of complex interdependence, as theorized by Keohane and Nye, where multiple channels of interaction, including economic, environmental, and diplomatic cooperation, reduce reliance on coercive military force.

Pluralist theory represents a further evolution of liberal thought, emphasizing that states are not the sole actors in international relations. Scholars such as R. Little, D. Nicholls, O.R. Young, and S. Hoffmann argue that individuals, groups, associations, and international organizations are crucial in shaping global politics. Pluralism aligns with sociological, institutional, and interdependence liberalism by highlighting the role of non-state actors, including NGOs and MNCs, in promoting cooperation, peace, and human development. The pluralist perspective counters state-centric realism, demonstrating that international relations encompass multiple, overlapping interactions with the potential to enhance stability and reduce conflict. Critics note that pluralism may underestimate state power and the influence of global inequalities, yet it provides a nuanced understanding of global interdependence and cooperation.

Despite its contributions, liberalism has faced persistent criticism. Realists such as E.H. Carr, Hans Morgenthau, and Kenneth Waltz argue that human nature is self-interested, and conflict is inevitable in an anarchic international system. Realists contend that cooperation is limited, and morality plays a secondary role in statecraft, where power and security dominate decision-making. Neo-Marxist critics argue that liberalism overlooks structural inequalities in the global system, perpetuating economic dependency, exploitation, and neocolonial hierarchies. Feminist scholars critique liberalism for neglecting gendered hierarchies, while postcolonial theorists highlight its Eurocentric assumptions, which may fail to account for the perspectives of the Global South. These critiques, however, have often catalyzed the refinement and diversification of liberal theory, leading to neo-liberal, pluralist, and institutional innovations that incorporate concerns of justice, equity, and transnational cooperation.

The relevance of liberalism in contemporary IR is evident in its influence on global governance, human rights, and institutional frameworks. In a world shaped by climate change, pandemics, cyber threats, and transnational terrorism, liberal frameworks emphasizing cooperation, rules, and multilateralism are essential for collective problem-solving. International institutions, trade regimes, human rights conventions, and multilateral treaties reflect the enduring legacy of liberal thought. Furthermore, liberalism informs foreign policy practices, from the European Union’s normative diplomacy to the United Nations’ peacekeeping operations, demonstrating its practical applicability. The liberal emphasis on interdependence, dialogue, and shared norms offers a counterweight to unilateralism and militarized approaches, reinforcing the role of diplomacy, negotiation, and multilateralism in maintaining international stability.

In conclusion, the evolution of liberal thought in IR traces a trajectory from philosophical individualism to institutionalized global cooperation, adapting to the complexities of changing international landscapes. From Lockean natural rights, Kantian perpetual peace, and Wilsonian idealism to neo-liberal institutionalism, interdependence theory, and pluralist approaches, liberalism provides a comprehensive framework for understanding, predicting, and shaping international relations. While subject to critiques from realism, Marxism, feminism, and postcolonial theory, liberalism’s emphasis on cooperation, institutions, law, and shared norms ensures its continued relevance. In an increasingly interconnected and interdependent world, liberal thought remains indispensable in promoting peace, stability, and human development, offering both a normative vision and practical mechanisms for navigating the challenges of contemporary international politics.

Idealism, often referred to as liberal internationalism, emerged prominently after the First World War, as scholars and policymakers sought to explain the causes of war and identify mechanisms to prevent future conflicts. At its core, idealism assumes that human nature is essentially rational and moral, and that individuals and states are capable of cooperation, ethical decision-making, and progress.

Thinkers such as Immanuel Kant, Woodrow Wilson, Norman Angell, and John Dewey emphasized that international relations are not governed solely by power politics but by norms, laws, and ethical considerations. Kant’s seminal work, “Perpetual Peace” (1795), argued that republican governments, adherence to international law, and economic interdependence could create conditions for lasting peace, highlighting the normative and aspirational dimensions of idealist thought. Similarly, Woodrow Wilson’s advocacy for the League of Nations exemplified the belief that collective security, multilateral diplomacy, and international institutions could mitigate conflict and promote global cooperation.

A fundamental assumption of idealism is that war is not inevitable, but rather a consequence of mismanagement, lack of communication, and absence of institutions to regulate interstate behavior. Unlike realists, idealists reject the notion of anarchy as an insurmountable condition, instead asserting that states and societies can cultivate norms, agreements, and ethical frameworks that constrain aggression. Idealism also assumes the importance of democracy, human rights, and economic interdependence in fostering peaceful international relations. For example, the Democratic Peace Theory, later articulated by scholars such as Michael Doyle, reflects idealist assumptions by suggesting that liberal democracies rarely go to war with each other due to shared values, institutional checks, and popular accountability.

The continuing relevance of idealism in peacebuilding is evident in the role of international institutions, treaties, and norms in resolving contemporary conflicts. Institutions such as the United Nations, International Court of Justice, and regional organizations like the African Union and ASEAN operationalize idealist principles by providing platforms for dialogue, negotiation, and conflict resolution. The effectiveness of peacekeeping missions, mediation efforts, and humanitarian interventions demonstrates that cooperation, law, and ethical considerations remain central to managing global disputes. Scholars such as Robert Keohane and Joseph Nye, through the lens of neo-liberal institutionalism, have reinforced the idealist insight that rules, institutions, and interdependence can reduce the likelihood of conflict, even in an anarchic international system.

Critics, particularly realists like Hans Morgenthau and E.H. Carr, argue that idealism is overly optimistic, underestimating the enduring role of power, self-interest, and strategic competition in international politics. They contend that institutions often reflect the interests of dominant states and may fail in crises, as seen in conflicts such as Syria or Ukraine, challenging the idealist assumption of universal cooperation.

Nonetheless, the normative and practical contributions of idealism remain significant, particularly in addressing non-traditional security threats, promoting human rights, and fostering multilateral solutions. In the contemporary context of globalization, climate change, and transnational conflicts, idealism provides a framework for peacebuilding strategies that integrate diplomacy, ethical norms, and cooperative mechanisms, illustrating its enduring analytical and policy relevance.